DEAFNESS |
BULL TERRIERS HEALTH UK |
How hearing works Sound waves enter the external ear canal and strike the ear drum, which begins to vibrate. This, in turn, causes vibration of the tiny bones in the middle ear (the ear ossicles). These vibrations set up waves in the fluid of the cochlea, the spiral-shaped structure in the inner ear. The change in pressure ultimately causes vibration of the hair cells inside the cochlea which are connected to the auditory nerve. This triggers a nerve impulse via the auditory pathway to the brain. Inherited Deafness Microscopic examination has shown that Inherited deafness is caused by degeneration of the blood supply to the cochlea in the first few weeks of life. This is followed by destruction of the hair cells and ultimately further deterioration of components of the inner ear and nerve degeneration. The loss of the hair cells is permanent and irreversible. The passage of sound to the auditory nerve is interrupted, resulting in the dog becoming deaf. Affected puppies can be detected by 4 weeks old and one study showed no evidence of progressive hearing impairment beyond this age. Deafness may occur in both ears (bilateral deafness) or in one ear only (unilateral deafness). Research in America has concluded that 1 in 6 white bull terriers unilaterally deaf (deaf in 1 ear) whilst 1 in 20 is bilaterally deaf (totally deaf). Not only white's but also coloured and even solid Bull Terrier can be deaf. Although it is possible to keep a bilaterally deaf dog, the problems associated with such a decision cannot be emphasised too highly. A deaf dog is an incredible responsibility and can be a liability both to itself and others. Deaf Bull Terriers should not be sold by breeders. Unilaterally deaf dogs make excellent pets, but they carry the genetic material for deafness. As the condition is hereditary, it makes sense not to use affected individuals for breeding. BAER TESTING The only way in telling the exact state of a dogs hearing is to BAER test (brainstem auditory evoked response). This method can distinguish between a totally deaf dog, a dog that is unilaterally deaf or a dog that can hear normally. When a sound enters the ear, tiny electrical impulses are generated by components of the auditory pathway in the inner ear and brain. These signals can be picked up by recording electrodes positioned on the head, and are in turn passed into a specialised electrodiagnostic machine. A series of approximately 500 stimuli, usually clicks, are passed into the ear through a headphone. In a normal-hearing dog a series of peaks and troughs is produced which is displayed on a small TV-type screen. Thus, the BAER can be defined as the electrical response of the brain to a series of auditory stimuli. The test can be carried out on puppies from 5 weeks old without any sedation, adult dogs usually need to be lightly sedated. More and more responsable UK breeders are starting to BAER test which will in the long term reduce the amount of deafness in the breed. Unfortunatly not many veterinary centers have the facilities to do BAER testing, see the list below; Animal Health Trust, Newmarket - 01638 552700 ChesterGates Referral Hospital - 01244 853823 Animal Medical Centre, Manchester - 0161 8813329 Hearing Assessment Clinic (mobile), Hampshire - 01962 713155 Vale Vets, Gloucester - 01453 542092 University of Glasgow Faculty of Veterinary Medicine, Tel: 0141 330 5848 Church Farm Veterinary Clince, Liverpool - 0151 3271885 Wey Referrals, Surrey - 01483 729194 |
References Julia Freeman, photo courtesy of Jo Gilbert (Icacai bull terriers) |
